Historically, a telephone service subscriber's directory number (colloquially referred to as the “telephone number”) identified the physical circuit or line that was connected to the customer's premises, one of many circuits or lines handled by a telephone switch. This physical circuit is also referred to as a switching port. The telephone switch, also referred to as a switching office, central office, or local exchange, is hereinafter referred to generically as a switching point or SP. Thus, a local telephone number “XXX-YYYY” indicated that the line, called the local loop, that connected the SP to the customer's premises was connected to switching port “YYYY” on switch “XXX”. When a subscriber changed locations, the subscriber's telephone number was also changed, representing a change from one switching port to another switching port, either in the same SP or a different SP. Since the telephone number of a called party corresponded to the physical circuit connecting the SP to the called party's premises, the telephone number itself was essentially the network address of the called party. For example, calling party A dials the telephone number of called party B, which is “123-4567”. The switch connected to calling party A's phone routes the call to local exchange 123; once the call reaches local exchange 123, the exchange routes the call to its own switching port 4567, which is physically connected to the local loop for called party B.
However, there are some situations where the correlation between a subscriber's directory number and a physical switch and port in a telephone exchange is no longer appropriate, in which case an additional layer of abstraction is needed to map the subscriber's directory number to specific hardware in a switch. One situation where a number translation function is needed involves number portability (NP), which refers to the ability of a telephone subscriber to retain the same directory number while changing geographic locations or service providers. Telephone number portability was mandated in the United States by the Telecommunications Act of 1996. In the number portability scenario, for example, if a subscriber has changed geographic locations and/or providers but wants to keep his or her current directory number, that subscriber may be added to a number portability lookup database that maps the subscriber to the network address of the switching point to which the subscriber has been ported.
There are other situations in which a number translation function is needed or useful. Switching office cutover (CTO), for example, refers to the situation where a subscriber has not changed geographic location or service provider, but is merely being migrated from one telephone exchange to another, such as when an old telephone exchange equipment or building is being replaced with a new building or equipment. In this situation, calls formerly directed to the old exchange must be redirected to the new exchange, e.g., by translating the subscriber's telephone number to the network address of the new exchange that now handles the subscriber.
Call offloading (CO) is another example of number translation, where a call that normally travels through a public switched telephone network (PSTN) is routed to another network, such as to a high-speed digital backbone. For example, the call may be rerouted to avoid a congested PSTN node or trunk. In such a case, the address of the intermediate routing nodes may be remapped from PSTN nodes to packet network nodes (and back) via gateway node(s) connecting both networks. Call offloading may redirect calls from SS7-based networks, such as PSTN, 2G, Wireless, GSM, IS-41, etc., to non-SS7-based networks, such as networks based on Internet protocol multimedia subsystem (IMS), next generation network (NGN), session initiation protocol (SIP), H.323, and others.
An advanced routing number (ARN) function is another example of number translation, where the destination of a call, for example, may be modified based on parameters and conditions. A call to a toll-free number, for example, may be mapped to the number of an east-coast or west-coast office, or even an overseas office, depending on the time of day and which office is open at the time of the call.
However, while number translation functions such as NP, CTO, CO, and ARN return the network address of the recipient SP, i.e., the switching point to which the call must be redirected, the recipient SP must further process the call to determine the final destination of the call, e.g., the specific line card/port within the SP. Typically, this requires an additional number translation or number lookup operation to determine a local directory number (LDN), which is private to the recipient switching office. Thus, while conventional number translation functions provide the address of the recipient SP, they do not provide enough information to completely identify the final destination of the call, causing a need for an additional database access. Performing two number translation operations is inefficient and time-consuming and requires that the recipient SP have the capability to perform a database or other datastore access, which increases the cost and complexity of the SP.
Accordingly, there exists a need for number translation with local directory number support.